1. Field of the Invention
This invention relates to solid state light emitting diodes (LEDs) and lasers that can emit various colors of light, including white.
2. Description of the Related Art
Light emitting diodes (LEDs) are an important class of solid state devices that convert electric energy to light. They generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted omnidirectionally from the active layer and from all surfaces of the LED. The useful light is generally emitted in the direction of the LED's top surface, which is usually p-type.
One disadvantage of conventional LEDs is that they cannot generate white light from their active layers. One way to produce white light from conventional LEDs is to combine different colors from different LEDs. For example, the light from red, green and blue LEDs, or blue and yellow LEDs can be combined to produce white light. One disadvantage of this approach is that it requires the use of multiple LEDs to produce a single color of light, increasing costs. In addition, different colors of light are often generated from different types of LEDs which can require complex fabrication to combine in one device. The resulting devices can also require complicated control electronics since the different diode types can require different control voltages. Long term wavelength and stability of these devices is also degraded by the different aging behavior of the different LEDs.
More recently, the light from a single blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye. [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc., which comprise blue LEDs surrounded by a yellow phosphor powder.; see also U.S. Pat. No. 5,959,316 to Hayden, entitled Multiple Encapsulation of Phosphor-LED Devices.] The surrounding material “downconverts” the wavelength of some of the LED light, changing its color. For example, if a nitride based blue emitting LED is surrounded by a yellow phosphor, some of the blue light will pass through the phosphor without being changed while the remaining light will be downconverted to yellow. The LED will emit both blue and yellow light, which combine to produce white light.
However, the addition of the phosphor results in a more complex LED that requires a more complex manufacturing process. In addition, the net light emitting efficiency is reduced due to the absorption in the phosphor and the stokes shift from blue to yellow. Other examples of LEDs using this approach include U.S. Pat. No. 5,813,753 to Vriens et al., and U.S. Pat. No. 5,959,316 to Lowery.
Another disadvantage of most conventional LEDs is that they are less efficient at converting current to light compared to filament lights. However, recent advances in nitride based LEDs have resulted in highly efficient blue light sources, and their efficiency is expected to surpass filament (and flourescent) based light sources. However, conventional blue LEDs operate from a relatively low supply current that results in a light that is too dim for many lighting applications. This problem is compounded by the absorption of some of the blue light by the downconverting material used to generate white light from blue. For blue LEDs to provide a bright enough light source for room illumination, the current applied to the LED must be increased from the conventional 20-60 mAmps to 0.8-1 Amp. At this current, LEDs become very hot and any material surrounding the LED will also become hot. The heat can damage the downconverting material surrounding the LED, degrading its ability to downconvert the LED's light. The heat can also present a danger of burning objects that are near to or in contact with the LED.
Another disadvantage of conventional LEDs is that they only emit one color of light. In conventional multi-color LED displays, different LEDs must be included to generate different colors of light. In applications such as displays or television screens, this can result in a prohibitive number of LEDs and can require complex control electronics.
Solid state lasers convert electrical energy to light in much the same way as LEDs. [Prentice Hall, Laser Electronics 2nd Edition, J. T. Verdeyen, Page 363 (1989)]. They are structurally similar to LEDs but have mirrors on two opposing surfaces. In the case of edge emitting lasers, the mirrors are on the device's side surfaces and reflect light generated by the active layer until it reaches a high enough energy level to escape from the side of the laser, through one of the mirrors. This results in a highly collimated/coherent light source. A vertical cavity laser works much the same as an edge emitting laser, but the mirrors are on the top and the bottom. Light from the active layer reflects between the mirrors until it reaches a stimulated emission level, providing a similar collimated light source from the laser's top surface.
However, conventional solid state lasers cannot efficiently emit green and blue light. Red emitting solid state lasers are more common, but their performance degrades with temperature and if the temperature reaches a high enough point, the laser will stop emitting light.